Future wireless communication systems will provide a wide range of services using a variety of access technologies. Such systems are often described as beyond third generation (B3G) systems and will include support for heterogeneous network access, communication service, user devices and mobility service. Modern networks are becoming increasingly diverse in their interconnectivity and transport capabilities, and B3G systems will need to be capable of managing even greater diversity. B3G systems may thus need to operate in an environment where almost all devices are networked, and almost all network entities are mobile.
B3G systems will first need to overcome many disadvantages associated with current technology. For example, in most wireless mobile communication systems a large frequency band is assigned to the whole system. The assigned frequency band is then shared by multiple users using multiple access techniques, such as Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). Both FDMA and CDMA systems operate in a frequency-selective propagation environment, where larger frequency diversity gain is obtained by assigning a wider frequency band to a user. However, FDMA systems allocate narrow frequency bands to low-rate users, and thus frequency diversity gain is not available to these users. CDMA systems provide a large frequency diversity gain, but suffer from Multiple Access Interference (MAI). Therefore, new B3G multiple access schemes, which provide both frequency diversity gain and orthogonality among multiple users, are desirable.
One B3G candidate technology is Interleaved Frequency Division Multiple Access (IFDMA). IFDMA is a scheme of orthogonal multiple access using spread spectrum techniques. A frequency band assigned to a system is divided into sub-carriers, and a different set of sub-carriers is assigned to each user. IFDMA provides frequency diversity gain by assigning multiple sub-carriers to a user while maintaining orthogonality between users.
In an IFDMA system, a mobile station requests a base station to activate a number of sub-carriers. The base station then activates a number of sub-carriers for the mobile station based on the requested number of sub-carriers. Generally, multiple mobile stations request such activation, and thus multiple sets of sub-carriers are activated simultaneously. However, all possible sub-carriers are not always activated. That is because the system bandwidth and sub-carrier spacing are generally constant, which means that the maximum number of sub-carriers is also constant. So when the total number of requested sub-carriers is smaller than the maximum number of sub-carriers, a base station will not need to activate all sub-carriers. But when all sub-carriers are not activated IFDMA systems are not always able to obtain full frequency diversity gain, resulting in sub-optimal performance.
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